Method and plant for heating a liquid medium

ABSTRACT

The invention relates to a method of heating a liquid medium by means of a first thermal system ( 2, 3, 4, 7, 12, 13 ) and at least one second thermal system ( 5, 6, 6 A,  15, 16 ) following said first thermal system ( 2, 3, 4, 7, 12, 13 ), which thermal systems each have at least one heat exchanger ( 2, 5 ) through which the medium flows, and which second thermal system ( 5, 6, 6 A,  15, 16 ) is operated at a higher temperature level than the first thermal system ( 2, 3, 4, 7, 12, 13 ). The method is characterized in that, for the accelerated raising of the temperature of the medium in the first thermal system ( 2, 3, 4, 7, 12, 13 ), the direct feed of the medium to the same is reduced and in the extreme case prevented, and in that medium flowing through the first thermal system ( 2, 3, 4, 7, 12, 13 ) is directed in a circuit. The invention also relates to a plant for carrying out the method.

[0001] The present invention relates to a method of heating a liquidmedium by means of a first thermal system and at least one secondthermal system following said first thermal system, which thermalsystems each have at least one heat exchanger through which the mediumflows, and which second thermal system is operated at a highertemperature level than the first thermal system. It also relates to aplant for carrying out the method, including a feed line for feeding themedium to be heated.

[0002] Plants in which a liquid medium passes through a plurality ofthermal systems in order to be heated, possibly evaporated, are present,for example, in boilers which are heated by flue gas from burners orexhaust gas from gas turbines.

[0003] The medium may be water, having additives if need be. Dependingon the final load, the water is heated in the boiler to a predeterminedtemperature in order to be fed, for example, to an industrial plant, ahot-water network, etc., or evaporated in order to be fed, for example,to a steam turbine or an industrial steam load.

[0004] The first thermal system in such a boiler, which has a first heatexchanger, a heating-area bank, is normally called the economizer. Dueto the temperature conditions, the economizer, which is provided forpreheating the feed water in the boiler, preferably works on theflue-gas-side or exhaust-gas-side end of the boiler, i.e. atcomparatively low temperatures.

[0005] On the other hand, the temperature difference between the fluegas or exhaust gas and the feed water to be heated is relatively small.This in turn results in large heating areas and large heating-areamasses associated therewith.

[0006] Consequently, an economizer requires a considerable amount oftime for adaptation of the temperature, for example during a change inthe operational conditions. Furthermore, it is known that there is arisk of dew-point corrosion on account of the temperatures and pressuresprevailing in the economizer.

[0007] Known methods of raising the feed-water temperature at the boilerinlet or for avoiding dewpoint corrosion at the flue-gas-side boilerend, for example as a function of the fuel used, are

[0008] recirculation and

[0009] bypassing the economizer.

[0010] In the case of recirculation, water preheated at the boiler inletis admixed with the feed water. For the bypassing of the economizer, thefeed water bypasses the economizer, and the preheating is carried out ina system working at a higher temperature level, for example asteam-generating system, at the cost of the reduction in the steamgeneration.

[0011] In order not to damage the heating areas, in particular duringthe start-up or during a change to a sulfurous fuel, measures which gobeyond the said measures, i.e. which permit markedly quicker temperatureraising in the economizer region, are necessary.

[0012] The object of the invention is therefore to provide a method ofheating a liquid medium by means of a first thermal system and a secondthermal system following said first thermal system and having a highertemperature level, according to which method accelerated raising of thetemperature of the first thermal system is made possible under specialoperating conditions (start-up, fuel change). Furthermore, the risk ofdew-point corrosion is to be reduced.

[0013] According to the invention, this is achieved in that, for theaccelerated raising of the temperature of the medium in the firstthermal system, the direct feed of the medium to the same is reduced andin the extreme case prevented, and in that medium flowing through thefirst thermal system is directed in a circuit.

[0014] A plant for carrying out the method according to the invention ischaracterized in that the first thermal system has a first heatexchanger, having an inlet line adjoining the feed line, and an outletline which runs via a line section to the second thermal system, a firstcontrol element being arranged between the feed line and the inlet line,in that a bypass line equipped with a second control element runs fromthe feed line to the outlet line, in that a line section runs from theoutlet line to the second thermal system, and in that the outlet line isconnected to the inlet line via a recirculation line having a thirdcontrol element and a first pump, which recirculation line is arrangedparallel to the first heat exchanger.

[0015] Advantageous developments of the invention are described in thesubclaims.

[0016] Various circuit arrangements for explaining various embodimentsof the invention are shown in a simplified form in the drawing figures.Only the elements essential for the understanding of the invention areshown.

[0017]FIG. 1 shows a circuit arrangement in a first embodiment of theinvention, having a drum circulation evaporator as second thermalsystem,

[0018]FIG. 2 shows a circuit arrangement identical to that of FIG. 1,but having a once-through evaporator as second thermal system,

[0019]FIG. 3 shows a circuit arrangement in a second embodiment of theinvention, having a drum circulation evaporator as second thermalsystem,

[0020]FIG. 4 shows a circuit arrangement in a third embodiment of theinvention, having a second preheating stage with tank as second thermalsystem,

[0021]FIG. 5 shows a circuit arrangement in a fourth embodiment of theinvention, having a once-through evaporator as second thermal system,

[0022]FIG. 6 shows a circuit arrangement in a fifth embodiment of theinvention, having a second preheating stage with tank as second thermalsystem,

[0023]FIG. 7 shows a circuit arrangement in a sixth embodiment of theinvention, having a drum circulation evaporator as second thermalsystem,

[0024]FIG. 8 shows a circuit arrangement in a seventh embodiment of theinvention, having a second preheating stage with tank as second thermalsystem,

[0025]FIG. 9 shows a circuit arrangement in an eighth embodiment of theinvention, having a drum circulation evaporator as second thermalsystem.

[0026] A section of a boiler is used as exemplary embodiment forexplaining the invention. This section is to have a first thermal systemand a second thermal system, the second thermal system being operated ata higher temperature level than the first thermal system.

[0027] In concrete terms, in the exemplary embodiments shown, the firstthermal system comprises the economizer and the second thermal systemcomprises the evaporator of the boiler. In this case, for the ideabehind the invention, it is irrelevant whether the evaporator is a drumcirculation evaporator or a once-through evaporator, as becomes apparentfrom the examples described below.

[0028] Further exemplary embodiments have a second preheating stage withtank as second thermal system.

[0029] The following figures, methods and explanations are in principlebased on one another.

[0030] Reference is now made to the first embodiment according to FIG. 1having a drum circulation evaporator as second thermal system. Thereference numeral 1 designates the feed-water line through which themedium to be heated, i.e. prepared feed water, is fed. The feed water isdelivered to the boiler by the feed-water pump 31. The feed-water line 1ends at a first control element 10. Downstream of the first controlelement 10, an inlet line 3 runs to a first heat exchanger 2 (theeconomizer), which is followed by an outlet line 4. The line section 9leads as an extension of the outlet line 4 to the second thermal system,in the actual case to the steam drum 6.

[0031] Upstream of the first control element 10, a bypass line 8 havinga second control element 11 branches off from the feed-water line 1 tothe outlet line 4.

[0032] A recirculation line 7 having a first pump 13 and a third controlelement 12 extends between the outlet line 4 and the inlet line 3, inwhich case it can be seen from the drawing figure that the first pump 13is arranged for delivery from the outlet line 4 to the inlet line 3.Downstream of the branching point of the recirculation line 7 from theoutlet line 4, a fourth control element 14 is arranged in the outletline 4.

[0033] The second thermal system comprises a second heat exchanger 5,the exemplary evaporator, which is connected to a tank for receiving aquantity of the medium in the liquid state, here in concrete terms to asteam drum 6. From the steam drum 6, a supply line 15 leads to thesecond heat exchanger 5. From the second heat exchanger 5, a return line16 leads to the steam drum 6. The reference numeral 32 designates anoutlet line of the steam drum 6, this outlet line leading, for example,to a steam load, a steam turbine, a superheater, etc.

[0034] The two heat exchangers 2, 5 are heated by a heating gas 56,which may be flue gas in the case of a boiler fired by burners orexhaust gas in the case of the waste-heat utilization of a gas turbine.

[0035] The heating of the heat exchangers 2, 5 is identical in all theexemplary embodiments and is therefore not explained again.

[0036] During normal operation of the two thermal systems, the firstcontrol element 10 and the fourth control element 14 are open, and thesecond control element 11 and the third control element 12 are closed.Furthermore, the first pump 13 is shut down.

[0037] The water flowing in the direction of arrow 33 through thefeed-water line 1 therefore flows through the inlet line 3 to the firstheat exchanger 2, the economizer, from the latter through the outletline 4 and its extension, the line section 9, into the steam drum 6 oralternatively into the supply line 15, as indicated by dash-lined arrow38.

[0038] From the steam drum 6, the water flows through the supply line 15to the second heat exchanger 5, the evaporator, and the steam or thewater/steam mixture flows from the second heat exchanger 5 through thereturn line 16 back to the steam drum 6. Water and steam are separatedin the steam drum 6. Finally, the steam flows through the outlet line 32to a load.

[0039] The circulation or pass in the second thermal system may beeffected by natural flow, by a pump or by a combination of both methods.

[0040] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the fourth control element 14 are at least partlyclosed—in the extreme case completely closed. The second control element11 and the third control element 12 are at least partly opened—in theextreme case completely opened. The pump 13 is in operation.

[0041] The water to be heated therefore flows in the circuit, in theextreme case, with control elements completely closed and openrespectively, in a completely closed circuit, in the direction of arrow34 from the cold end to the hot end of the first heat exchanger 2,through the outlet line 4 to the recirculation line 7, flows in thedirection of arrow 35 through the same, then to the inlet line 3 andfinally back to the cold end of the first heat exchanger 2.

[0042] Consequently, unused and thus “cold” feed water is not constantlyfed to the cold end of the heat exchanger 2 via the feed-water line 1,but rather heated water already flowing in from the hot end of the heatexchanger 2 is fed to the cold end. Quicker heating not only of thewater but also of the heating-area mass of the first heat exchanger 2 isthus effected.

[0043] Since the control elements 10, 14 are closed and the controlelement 11 of the bypass line 8 is in the open position, the water flowsfrom the feed-water line 1 in the direction of arrow 36 through thebypass line 8 and the line section 9 to the second thermal system.

[0044] In this case, two variants are possible:

[0045] As indicated by arrow 37, the water can flow via the line section9 directly into the steam drum 6.

[0046] As indicated by dash-lined arrow 38, the water can flow via theline section 9 into the supply line 15.

[0047] For reasons of clarity, any fittings assigned to the flowvariants according to arrows 37 or 38 are not shown.

[0048] It may be noted that the respective control elements need notnecessarily be in a completely closed or completely open position.Intermediate positions are also possible in order to achieve the bestpossible effect. Controlled movements from one position into the otherposition are also envisaged, for example in order to avoid thermalshocks.

[0049] With regard to the first thermal system, the exemplary embodimentshown in FIG. 2 is identical to the exemplary embodiment according toFIG. 1.

[0050] The second thermal system is a once-through evaporator,consisting of the second heat exchanger, the evaporator 5, the supplyline 15 and return line 16 connected to the evaporator 5, and aseparator 6A.

[0051] Unlike the variant of a drum circulation evaporator shown in FIG.1, the flow through the second thermal system takes place through theline section 9 into the supply line 15, in the direction of arrow 39through the evaporator 5 and via the return line 16 into the separator6A.

[0052] Water and steam are separated in the separator 6A. The steamflows via the outlet line 32 to a steam load or superheater. The waterseparated in the separator is fed back to the evaporator 5 via thesupply line 15 having the circulation pump 40.

[0053] The operation of the first thermal system then, both duringnormal operation and during the operation for the accelerated raising ofthe temperature, is completely identical to that according to FIG. 1.

[0054] A second embodiment of the invention is described below withreference to FIG. 3, in which case, as an embodiment variant, a drumcirculation evaporator having the steam drum 6 is again used as secondthermal system. As far as possible, the same reference numerals as inFIGS. 1 and 2 are used.

[0055] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, a fourth control valve 14, and a recirculation line 7 having afirst pump 13 and a third control element 12.

[0056] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. The line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the steam drum 6. Thesecond thermal system has, in particular, a steam drum 6 with an outletline 32 and a second heat exchanger 5, which is connected to the steamdrum 6 via a supply line 15 and a return line 16.

[0057] The difference between this second embodiment according to FIG. 3and the embodiment according to FIGS. 1 and 2 lies in the arrangement ofthe pump 13 in the recirculation line 7.

[0058] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the fourth control element 14 are at least partlyclosed—in the extreme case completely closed. The second control element11 and the third control element 12 are at least partly opened—in theextreme case completely opened—and the pump 13 is put into operation.

[0059] In this embodiment, the water to be heated flows in the circuitin the direction of arrow 43 from the hot end to the cold end of thefirst heat exchanger 2, through the inlet line 3 to the recirculationline 7, flows in the direction of arrow 42 through the latter, then tothe outlet line 4 and finally back to the hot end of the first heatexchanger 2.

[0060] The flow in the second thermal system according to FIG. 3 is thesame as the flow in the second thermal system of the embodimentaccording to FIG. 1.

[0061]FIG. 4 shows a third embodiment, a second preheating stage havinga second heat exchanger 5 and a tank 6 being used as embodiment variantfor the second thermal system. As far as possible, the same referencenumerals as in the preceding embodiments have been used.

[0062] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, a fourth control valve 14, and a recirculation line 7 having afirst pump 13 and a third control element 12.

[0063] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. The line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the tank 6. The secondthermal system has, in particular, a tank 6 with an outlet line 32 and asecond heat exchanger 5, which is connected to the tank 6 via a supplyline 15 and a return line 16.

[0064] The feed water flowing into the tank 6 via the line section 9 inthe direction of arrow 37 may alternatively also flow into the supplyline 15, as indicated by dash-lined arrow 38.

[0065] In this embodiment, a line 17 having a pump 18 and a controlelement 19 runs from the second heat exchanger 5 to the hot end of thefirst heat exchanger 2 or to the outlet line 4.

[0066] In a second variant, which is depicted by a dashed line, a line41, which merges into the line 17, branches off from the supply line 15.

[0067] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10, the second control element 11 and the controlelement 19 are in the open position. The third control element 12 andthe fourth control element 14 are in the closed position. The first pump13 in the recirculation line 7 is shut down.

[0068] The feed water flowing in through the feedwater line 1 in thedirection of arrow 33 flows through the bypass line 8 in the directionof arrow 36 and through the line section 9 directly into the secondthermal system, either into the tank 6, as shown by arrow 37, oralternatively into the supply line 15, as shown by dash-lined arrow 38.

[0069] From the second heat exchanger 5, the water flows in thedirection of arrow 51 through the line 17 into the outlet line 4 and tothe hot end of the first heat exchanger 2. Furthermore, the water flowsin the direction of arrow 43 from the hot end to the cold end of thefirst heat exchanger 2 and then to the inlet line 3.

[0070] At the end of the inlet line 3, this water flow, as shown byarrow 44, is mixed with the feedwater flow flowing in through thefeed-water line 1, whereupon both water flows flow together through thebypass line 8 and the line section 9 to the second thermal system, i.e.to the tank 6 or to the supply line 15. Thus a circuit comprising boththermal systems is formed.

[0071] In the second variant, water can flow out of the supply line 15via the line 41 into the line 17.

[0072] It is now also possible to run the plant according to FIG. 4 inaccordance with the plant according to FIG. 1 by the control elements10, 14, 19 being closed, the control elements 11 and 12 being open, thefirst pump 13 being put into operation and the pump 18 being shut down.

[0073] That is to say that it is possible with this embodiment to carryout a start-up in two phases, namely during a first phase according tothe method which is possible with the arrangement according to FIG. 1,and during a second phase according to the method which is possible withthe arrangement described first according to FIG. 4, or vice versa.

[0074] This is intended to illustrate that the exemplary embodimentsdescribed may of course also be used in any desired combinations.

[0075]FIG. 5 shows a circuit arrangement in a fourth embodiment of theinvention. This arrangement, in accordance with the exemplary embodimentaccording to FIG. 2, has a once-through evaporator as second thermalsystem.

[0076] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which is of identical design to the first thermalsystems described above and has, in particular, a first heat exchanger 2having an inlet line 3 and an outlet line 4, a first control element 10,a fourth control element 14, and a recirculation line 7 having a firstpump 13 and a third control element 12.

[0077] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. The line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the supply line 15. Thesecond thermal system has, in particular, a second heat exchanger, theevaporator 5, to which feed water is admitted via a supply line 15 andwhich is connected to the separator 6A via the return line 16.

[0078] The flow through the second thermal system takes place throughthe line section 9 into the supply line 15, in the direction of arrow 39through the evaporator 5 and via the return line 16 into the separator6A.

[0079] Water and steam are separated in the separator 6A. The steamflows via the outlet line 32 to a steam load or superheater. The waterseparated in the separator is fed back to the evaporator 5 via thesupply line 15 having the circulation pump 40.

[0080] From the separator 6A, a line 20 having a further pump 21 and afurther control element 22 runs to the outlet line 4, in particular tothe hot end of the first heat exchanger 2.

[0081] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the thirdcontrol element 12 and the fourth control element 14 are closed. Thefirst pump 13 in the recirculation line 7 is not in operation.

[0082] The first control element 10 in the inlet line 3, the secondcontrol element 11 in the bypass line 8 and the control element 22 inthe line 20 are in the open position; the pump 21 is in operation.

[0083] The feed water therefore flows from the feedwater line 1 throughthe bypass line 8 and the line section 9 in the direction of arrow 36into the supply line 15 and thus to the second thermal system.

[0084] From the separator 6A, water now flows in the direction of arrow46 through the line 20 to the outlet line 4, i.e. to the hot end of thefirst heat exchanger 2. Furthermore, the water flows in the direction ofarrow 43 through the first heat exchanger 2 to its cold end, then in thedirection of arrow 44 through the inlet line 3 to the bypass line 8 inorder to flow back with feed water to the second thermal system.

[0085] In this embodiment, there is therefore a circuit comprising boththermal systems.

[0086]FIG. 6 shows a fifth embodiment, a second preheating stage havinga second heat exchanger 5 and a tank 6 being used as embodiment variantfor the second thermal system.

[0087] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, and a recirculation line 7 having a first pump 13 and a thirdcontrol element 12.

[0088] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. A line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the tank 6. The secondthermal system has, in particular, a tank 6 with an outlet line 32 and asecond heat exchanger 5, which is connected to the tank 6 via a supplyline 15 and a return line 16.

[0089] The feed water flowing into the tank 6 via the line section 9 inthe direction of arrow 37 may alternatively also flow into the supplyline 15, as indicated by dash-lined arrow 38.

[0090] In this embodiment, a line 23 having a pump 24 and a controlelement 25 runs from the second heat exchanger 5 to the cold end of thefirst heat exchanger 2 or to the inlet line 3.

[0091] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the third control element 12 are in the closedposition, and the first pump 13 is shut down. The second control element11 and the control element 25 are in the open position, and the pump 24is in operation.

[0092] In this embodiment, the water flows in the direction of arrow 51,through the line 23, further in the direction of arrows 47, 34 and 48through the first heat exchanger 2 and then together with the feedwater, flowing in via the feed-water line 1 and the bypass line 8, viathe line section 9, in a first variant, in the direction of arrow 37into the tank 6 or, in a second variant, in the direction of dash-linedarrow 38 into the supply line 15.

[0093]FIG. 7 shows a circuit arrangement in a sixth embodiment of theinvention, having a drum circulation evaporator with the steam drum 6 assecond thermal system.

[0094] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, and a recirculation line 7 having a first pump 13 and a thirdcontrol element 12.

[0095] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. A line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the steam drum 6. Thesecond thermal system has, in particular, a steam drum 6 with an outletline 32 and a second heat exchanger 5, which is connected to the steamdrum 6 via a supply line 15 and a return line 16.

[0096] The feed water flowing into the steam drum 6 via the line section9 in the direction of arrow 37 may alternatively also flow into thesupply line 15, as indicated by dash-lined arrow 38.

[0097] In a first variant, a line 26 having a pump 27 and a controlelement 28 runs from the steam drum 6 to the cold end of the first heatexchanger 2 or to the inlet line 3.

[0098] In a second variant, which is depicted by a dashed line, a line45, which merges into the line 26, branches off from the supply line 15.

[0099] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the third control element 12 are closed. Thefirst pump 13 in the recirculation line 7 is shut down.

[0100] The second control element 11 in the bypass line 8 and thecontrol line 28 in the line 26 are in the open position, and the pump 27is in operation.

[0101] The feed water therefore flows from the feedwater line 1 throughthe bypass line 8 and the line section 9 in the direction of arrows 36and 37 into the steam drum 6 or alternatively into the supply line 15,as indicated by dash-lined arrow 38.

[0102] From the steam drum 6, water now flows in the direction of arrow46 through the line 26 having the pump 27 and the control element 28 tothe inlet line 3, i.e. to the cold end of the first heat exchanger 2, inthe direction of arrows 47, 34 to the hot end of the first heatexchanger 2, and through the outlet line 4 in the direction of arrow 48to the line section 9 in order to flow together with the feed waterflowing in directly to the steam drum 6 or into the supply line 15.

[0103] In a second variant, water can flow out of the supply line 15 viathe line 45 into the line 26.

[0104] In this embodiment, there is therefore a circuit comprising boththermal systems.

[0105]FIG. 8 shows a circuit arrangement in a seventh embodiment of theinvention, a second preheating stage having a second heat exchanger 5and a tank 6 being used as embodiment variant for the second thermalsystem.

[0106] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, and a recirculation line 7 having a first pump 13 and a thirdcontrol element 12.

[0107] A further control element 49 is arranged in the recirculationline 7.

[0108] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. The line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the tank 6. The secondthermal system has, in particular, a tank 6 with an outlet line 32 and asecond heat exchanger 5, which is connected to the tank 6 via a supplyline 15 and a return line 16.

[0109] The feed water flowing into the tank 6 via the line section 9 inthe direction of arrow 37 may alternatively also flow into the supplyline 15, as indicated by dash-lined arrow 38.

[0110] In this embodiment, a line 29, in which a control element 50 isinserted, branches off from the second heat exchanger 5 and opens intothe recirculation line 7 at a point between the control element 49 andthe first pump 13.

[0111] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the control element 49 are closed. The controlelements 11, 12 and 50 are in the open position, and the first pump 13is put into operation.

[0112] The feed water therefore flows from the feedwater line 1 throughthe bypass line 8 and the line section 9 in the direction of arrows 36and 37 into the steam drum 6 or alternatively into the supply line 15,as indicated by dash-lined arrow 38.

[0113] From the second heat exchanger 5, water now flows in thedirection of arrow 51 through the line 29 into the recirculation line 7and in the direction of arrow 52 to the inlet line 3, the cold end ofthe first heat exchanger 2, through the first heat exchanger 2 to theoutlet line 4, the hot end of the first heat exchanger 2, to the linesection 9 and together with the feed water, flowing in directly throughthe bypass line 8, according to arrow 37 into the tank 6 oralternatively according to dash-lined arrow 38 into the supply line 15.

[0114] The direction of flow through the first heat exchanger 2 is shownby arrows 47, 34 and 48.

[0115]FIG. 9 shows a circuit arrangement in an eighth embodiment of theinvention, having a drum circulation evaporator with the steam drum 6 assecond thermal system.

[0116] The feed-water line 1 having the feed-water pump 31, throughwhich the feed water flows in the direction of arrow 33, runs to thefirst thermal system, which again has, in particular, a first heatexchanger 2 having an inlet line 3 and an outlet line 4, a first controlvalve 10, and a recirculation line 7 having a first pump 13 and a thirdcontrol element 12.

[0117] A further control element 49 is arranged in the recirculationline 7.

[0118] A bypass line 8 having a second control element 11 branches offfrom the feed-water line 1, which bypass line 8 runs to the outlet line4. A line section 9 leads as an extension of the outlet line 4 to thesecond thermal system, in the actual case to the steam drum 6. Thesecond thermal system has, in particular, a steam drum 6 with an outletline 32 and a second heat exchanger 5, which is connected to the steamdrum 6 via a supply line 15 and a return line 16.

[0119] The feed water flowing into the steam drum 6 via the line section9 in the direction of arrow 37 may alternatively also flow into thesupply line 15, as indicated by dash-lined arrow 38.

[0120] In a first variant, a line 54, in which a control element 55 isinserted, runs from the steam drum 6 to the recirculation line 7 andopens into the recirculation line 7 at a point between the furthercontrol element 49 and the first pump 13.

[0121] In a second variant, which is shown by a dashed line, a line 30,which merges into the line 54, branches off from the supply line 15.

[0122] For the accelerated raising of the temperature in the first heatexchanger 2, for example during the start-up of the plant, the firstcontrol element 10 and the control element 49 are closed. The controlelements 11, 12 and 55 are in the open position, and the first pump 13is put into operation.

[0123] The feed water therefore flows from the feedwater line 1 throughthe bypass line 8 and the line section 9 in the direction of arrows 36and 37 into the steam drum 6 or alternatively into the supply line 15,as indicated by dash-lined arrow 38.

[0124] From the steam drum 6, water now flows in the direction of arrow53 through the line 54 into the recirculation line 7 and in thedirection of arrow 52 to the inlet line 3, the cold end of the firstheat exchanger 2, through the first heat exchanger 2 to the outlet line4, the hot end of the first heat exchanger 2, to the line section 9 andtogether with the feed water, flowing in directly through the bypassline 8, according to arrow 37 into the steam drum 6 or alternativelyaccording to dash-lined arrow 38 into the supply line 15.

[0125] In a second variant, water can flow out of the supply line 15 viathe line 30 into the line 54.

[0126] The direction of flow through the first heat exchanger 2 is shownby arrows 47, 34 and 48.

[0127] The methods described can of course also be used in any desiredcombinations and chronological sequences.

[0128] The invention is in principle independent of the actual design,type of construction, structure and the like of the elements and systemsdescribed.

1. A method of heating a liquid medium by means of a first thermalsystem (2, 3, 4, 7, 12, 13) and at least one second thermal system (5,6, 6A, 15, 16) following said first thermal system (2, 3, 4, 7, 12, 13),which thermal systems each have at least one heat exchanger (2, 5)through which the medium flows, and which second thermal system (5, 6,6A, 15, 16) is operated at a higher temperature level than the firstthermal system (2, 3, 4, 7, 12, 13), characterized in that, for theaccelerated raising of the temperature of the medium in the firstthermal system (2, 3, 4, 7, 12, 13), the direct feed of the medium tothe same is reduced and in the extreme case prevented, and in thatmedium flowing through the first thermal system (2, 3, 4, 7, 12, 13) isdirected in a circuit.
 2. The method as claimed in claim 1 ,characterized in that the medium to be heated, which flows into thethermal systems (2, 3, 4, 7, 12, 13; 5, 6, 6A, 15, 16), is partly fed,and in the extreme case completely fed, directly to the second thermalsystem (5, 6, 6A, 15, 16).
 3. The method as claimed in claim 1 ,characterized in that the medium is directed in a circuit exclusively inthe first thermal system (2, 3, 4, 7, 12, 13).
 4. The method as claimedin claim 1 , characterized in that the medium is directed in a circuitin a combined manner in the first thermal system (2, 3, 4, 7, 12, 13)and in the second thermal system (5, 6, 6A, 15, 16).
 5. The method asclaimed in claim 1 , characterized in that the medium is directed in acircuit in the first thermal system (2, 3, 4, 7, 12, 13) from the coldend of its heat exchanger (2) to its hot end.
 6. The method as claimedin claim 1 , characterized in that the medium is directed in a circuitin the first thermal system (2, 3, 4, 7, 12, 13) from the hot end of itsfirst heat exchanger (2) to its cold end.
 7. The method as claimed inclaim 4 , characterized in that the medium is directed in a circuit fromthe second thermal system (5, 6, 6A, 15, 16) to the first thermal system(2, 3, 4, 7, 12, 13) and back to the second thermal system (5, 6, 6A,15, 16) and a direct feed of the medium to the second thermal system (5,6, 6A, 15, 16) is produced, in that the medium flowing out from thefirst thermal system (2, 3, 4, 7, 12, 13) is fed to the second thermalsystem (5, 6, 6A, 15, 16) together with the medium flowing in directlyto the second thermal system (5, 6, 6A, 15, 16), and an excess quantityof the medium is drawn off from the second thermal system (5, 6, 6A, 15,16).
 8. The method as claimed in claim 7 , characterized in that themedium is fed from the second thermal system (5, 6, 6A, 15, 16) to thehot end of the first heat exchanger (2) of the first thermal system (2,3, 4, 7, 12, 13).
 9. The method as claimed in claim 8 , characterized inthat the medium is fed from the second heat exchanger (5) of the secondthermal system (5, 6, 6A, 15, 16) to the hot end of the first heatexchanger (2) of the first thermal system (2, 3, 4, 7, 12, 13).
 10. Themethod as claimed in claim 8 , the second thermal system (5, 6, 6A, 15,16) having a tank (6, 6A) for receiving a quantity of the medium in aliquid state, characterized in that the medium is fed from the tank (6,6A) of the second thermal system (5, 6, 6A, 15, 16) to the hot end ofthe first heat exchanger (2) of the first thermal system (2, 3, 4, 7,12, 13).
 11. The method as claimed in claim 7 , characterized in thatthe medium is fed from the second thermal system (5, 6, 6A, 15, 16) tothe cold end of the first heat exchanger (2) of the first thermal system(2, 3, 4, 7, 12, 13).
 12. The method as claimed in claim 11 ,characterized in that the medium is fed from the second heat exchanger(5) of the second thermal system (5, 6, 6A, 15, 16) to the cold end ofthe first heat exchanger (2) of the first thermal system (2, 3, 4, 7,12, 13).
 13. The method as claimed in claim 11 , the second thermalsystem (5, 6, 6A, 15, 16) having a tank (6, 6A) for receiving a quantityof the medium in a liquid state, characterized in that the medium is fedfrom the tank (6, 6A) of the second thermal system (5, 6, 6A, 15, 16) tothe cold end of the first heat exchanger (2) of the first thermal system(2, 3, 4, 7, 12, 13).
 14. The method as claimed in claim 7 , the firstthermal system (2, 3, 4, 7, 12, 13) having a recirculation line (7)running from the hot end to the cold end of its first heat exchanger(2), characterized in that the medium is fed from the second thermalsystem (5, 6, 6A, 15, 16) via the recirculation line (7) to the cold endof the first heat exchanger (2) of the first system (2, 3, 4, 7, 12,13).
 15. The method as claimed in claim 14 , characterized in that themedium is fed from the second heat exchanger (5) of the second thermalsystem (5, 6, 6A, 15, 16) to the recirculation line (7) of the firstthermal system (2, 3, 4, 7, 12, 13).
 16. The method as claimed in claim14 , the second thermal system (5, 6, 6A, 15, 16) having a tank (6, 6A)for receiving a quantity of the medium in a liquid state, characterizedin that the medium is fed from the tank (6, 6A) of the second thermalsystem (5, 6, 6A, 15, 16) to the recirculation line (7) of the firstthermal system (2, 3, 4, 7, 12, 13).
 17. A plant for carrying out themethod as claimed in claim 1 , having a feed line (1) for feeding themedium to be heated, characterized in that the first thermal system (2,3, 4, 7, 12, 13) has a first heat exchanger (2), having an inlet line(3) adjoining the feed line (1), and an outlet line (4), a first controlelement (10) being arranged between the feed line (1) and the inlet line(3), in that a bypass line (8) equipped with a second control element(11) runs from the feed line (1) to the outlet line (4), in that a linesection (9) runs from the outlet line (4) to the second thermal system(5, 6, 6A, 15, 16), and in that the outlet line (4) is connected to theinlet line (3) via a recirculation line (7) having a third controlelement (12) and a first pump (13), which recirculation line (7) isarranged parallel to the first heat exchanger (2).
 18. The plant asclaimed in claim 17 , characterized in that the second thermal system(5, 6, 6A, 15, 16) has a second heat exchanger (5) and a tank (6, 6A)for receiving a quantity of the medium in the liquid state, which secondheat exchanger (5) is connected to the tank (6, 6A) via a supply line(15) running from the tank (6, 6A) to the second heat exchanger (5) andvia a return line (16) running from the second heat exchanger (5) backto the tank (6, 6A).
 19. The plant as claimed in claim 18 ,characterized in that a fourth control element (14) is arranged in theoutlet line (4) downstream of the point at which the recirculation line(7) branches off from the outlet line (4).
 20. The plant as claimed inclaim 18 , characterized in that the line section (9) runs to the tank(6, 6A) of the second thermal system (5, 6, 6A, 15, 16).
 21. The plantas claimed in claim 18 , characterized in that the line section (9)opens into the supply line (15).
 22. The plant as claimed in claim 18 ,characterized in that a circulation pump (40) is arranged in the supplyline (15).
 23. The plant as claimed in claim 17 , characterized in thatthe first pump (13) arranged in the recirculation line (7) is designedfor delivery of the medium from the outlet line (4) to the inlet line(3).
 24. The plant as claimed in claim 17 , characterized in that thefirst pump (13) arranged in the recirculation line (7) is designed fordelivery of the medium from the inlet line (3) to the outlet line (4).25. The plant as claimed in claim 18 , characterized in that a line (17)opening into the outlet line (4) and having a pump (18) delivering inthe direction of the outlet line (4) and a control element (19) isbranched off from the second heat exchanger (5).
 26. The plant asclaimed in claim 18 , characterized in that a line (20) opening into theoutlet line (4) and having a further pump (21) and a further controlelement (22) is branched off from the tank (6, 6A).
 27. The plant asclaimed in claim 18 , characterized in that a line (41, 17) opening intothe outlet line (4) and having a pump (18) delivering in the directionof the outlet line (4) and a control valve (19) is branched off from thesupply line (15).
 28. The plant as claimed in claim 18 , characterizedin that a line (23) opening into the inlet line (3) and having a pump(24) delivering in the direction of the inlet line (3) and a controlelement (25) is branched off from the second heat exchanger (5).
 29. Theplant as claimed in claim 18 , characterized in that a line (26) openinginto the inlet line (3) and having a pump (27) delivering in thedirection of the inlet line (3) and a control element (28) is branchedoff from the tank (6, 6A).
 30. The plant as claimed in claim 18 ,characterized in that a line (45, 26) opening into the inlet line (3)and having a pump (27) delivering in the direction of the inlet line (3)and a control element (28) is branched off from the supply line (15).31. The plant as claimed in claim 18 , characterized in that a line (29)opening into the recirculation line (7) and having a control element(50) is branched off from the second heat exchanger (5).
 32. The plantas claimed in claim 18 , characterized in that a line (54) opening intothe recirculation line (7) and having the control element (55) isbranched off from the tank (6, 6A).
 33. The plant as claimed in claim 18, characterized in that a line (30, 54) opening into the recirculationline (7) and having the control element (55) is branched off from thesupply line (15).